Two principles applies: Arquimede's and vacuum forces. The chain tensioner is connected to the oil circuit via narrow channel in the cranckcase (California models have their own oil pipe externally) and bleeding occurs once the oil pressure have been established via a relief valve. (When installing a new tensioner it has to be filled with oil prior to instal). As long as there is oil in the sump not far below the lower mark on the dipstick the cranckcase channels is blocked by the weight of the oil in the sump and the tensioner oil is not emptied due to the two above principles.
(Not rocket sciencie!).

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